We've all done it, for example - queuing as a Druid to force a high possibility to have a Druid on the enemy team, then swapping character at the last second to either counter that class or the team composition.
The other thing I've seen, is "top players" duo queuing as double Necromancer to force either one or two on the enemy team then swapping to double Revenant to counter and steamroll the game.
In a way this is kind of match manipulation as you're abusing how the matching making works.
Simple resolution, the class you queue as, is the class you play and are not allowed to swap once in the game.
Also double stacking classes should not be allowed to queue, unless ArenaNet fixes their algorithm.